In vascular plants, there is a clear coupling between traits related to water and traits related to carbon economics. For bryophytes this coupling has been little studied but is expected to be strong, because in these poikilohydric plants photosynthesis varies strongly with water availability. We hypothesized that there is a trade-off between water-holding and photosynthetic capacities for mosses, resulting in a limited spectrum of possible trait combinations. At one end of this spectrum, mosses would tend to stay wet and active for long periods but would have slow photosynthetic rates. At the other end, mosses would avoid external water and dry out quicker but would have high photosynthetic capacities. We determined the water relations (water-holding and -retention capacities), photosynthetic water- and light-response curves of shoots of 12 moss species and explored the associations between these traits and their distributions among the studied species. The results partly support our hypotheses, in that the water-holding and water-retention capacities of mosses are positively related to each other and to the value and width of the optimal water-content range for photosynthesis. However, the photosynthetic capacities were specific to taxonomic groups, and the relationships between the water relations and the photosynthetic capacity are weak or inconsistent and depend strongly on the species used for analysis. The positive relationships between water-holding, water-retention and photosynthetic water-use capacities suggest two contrasting adaptations to avoid damage during dehydration: taking more time to 'prepare' or quick photosynthetic adjustment. However, the spectrum we hypothesized cannot be generalized for all mosses and defining a broader spectrum will require the extension of this study to a much larger number of species and including stand-level measurements of water loss and photosynthesis.
Keywords: Bryophytes; Polytrichaceae; Sphagnum; gas exchange; poikilohydric; trade-offs; water and carbon economics.